The present invention relates to a superconducting crystal and a process for preparing the same.
Since the discovery of a high-temperature oxide superconductor in recent years, the research and development of a superconductor for practical use have actively been conducted in various application fields.
For example, a technique for forming a thin film of a superconducting oxide material is necessary for devices and elemental devices of computers and the like where use is made of superconducting quantum interference devices (SQUID) or Josephson devices. It has been reported in this relation that it is possible to prepare, through sputtering, vapor deposition, CVD, etc., a thin film of a single crystal free from grain boundary and having characteristics capable of sufficiently rendering the superconductor practicable, i.e., a critical current density exceeding about 10.sup.6 A/cm.sup.2.
Meanwhile, in order to take advantage of the characteristics of an oxide superconductor in power storage, power transportation, generation of a strong magnetic field, etc., it is necessary to fabricate wires therefrom. For this purpose, attempts have been made on, for example, a method wherein a calcined powder of an oxide superconductor is packed in a silver sheath and then again heat-treated, a sol-gel method, a method wherein a powder of an oxide superconductor is suspended in a polymer solution and wires are drawn from the suspension, and a method wherein wires are drawn from a molten oxide superconductor.
Each of the wires prepared by the above-described methods is a polycrystalline substance and has a low density and a grain boundary, so that no wire having characteristics on a practical level has been prepared as yet. Further, the above-described wires have drawbacks inherent in the polycrystalline, substance such as brittleness, poor fabricability, low bending strength, etc.
It has been reported that certain oxide super-conductors have each a critical temperature above the liquid nitrogen temperature, which has a significant meaning in connection with a cooling cost. Examples of the oxide superconductor having a critical temperature above the liquid nitrogen temperature include those based on bismuth, yttrium and thalium.
For the Bi-based oxide, there are three types of structures, i.e., Bi.sub.2 Sr.sub.2 Cu.sub.1 O.sub.6 structure (2201 structure), Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8 structure (2212 structure) and Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10 structure (2223 structure) respectively called the 20K structure, 80K structure and 110K structure according to their critical temperatures. In particular, the 2223 structure having the highest critical temperature has been regarded as a material best suited for practical use from the viewpoint of a possible wide margin between the critical temperature and the liquid nitrogen temperature. In fact, however, it is difficult to obtain a single phase of the 2223 structure, and the 2223 structure tends to form a mixed phase with the 2212 or 2201 structure.
According to the solid reaction, a single phase of the 2223 structure can be obtained under strict control of the composition and heating atmosphere. The resultant product, however, is a powdery or polycrystalline sinter and has a low density and many grain boundaries.
Therefore, as with the 2201 and 2212 structures wherein a large single crystal, single-crystal fibrous crystal, etc. have already been obtained, the formation of a large single crystal, single-crystal fibrous crystal, etc., of the 2223 structure would be very useful from the viewpoint of practical use.
In view of the above-described state of the art, the present inventors have made various studies and, as a result, have found that the heat treatment of a fibrous crystal or a single crystal having a Bi.sub.2 Sr.sub.2 Cu.sub.1 O.sub.6 structure or a Bi.sub.2 Sr.sub.2 Ca.sub.l Cu.sub.2 O.sub.8 structure in an oxide powder having a particular composition containing bismuth, strontium, calcium, copper and lead under particular conditions causes the crystal structure to be converted into a Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10 structure (2223 structure) while maintaining the original outline of fibrous crystal or the original outline of single crystal, and raises the critical temperature above the liquid oxygen temperature, which has led to the completion of the present invention.